1. Field of the Invention
The present invention relates to a joint structure of a flat cable (more specifically, flexible flat cable (FFC)) in a signal transmission device for a vehicle steering mechanism and joint terminals.
2. Description of the Prior Art
Generally, a signal transmission device for a vehicle steering mechanism is used to transfer a signal between a rotary portion which is rotated with a steering wheel and a fixed portion still regardless of the rotation of the steering wheel. In order to transfer the signal from the rotary portion to the external fixed portion, the first end of a flat cable is connected to the rotary portion whereas the other end thereof is connected to the fixed portion through a connector. The second end of the flat cable is connected to the connector in such a manner that the second end of the flat cable is connected to the first ends of joint terminals and the second ends of the joint terminals are inserted into the connector.
A previously known joint structure a of a flat cable and joint terminals is such as shown in FIG. 7.
As seen from FIG. 7, a plurality of joint terminals b include first areas b1 which are terminal plates and second areas b2 which are connector terminals. Conductors c2 exposed by stripping the coating of an insulating coating layer c1 from a flat cable c is welded to the terminal plates b1 by e.g. ultrasonic welding. The connector terminal areas b2 are secured within a terminal chamber d for the connector. A region e where the terminal plates b1 and the conductors c2 are connected is molded by insulating resin. This mold structure is sandwiched between and held by an upper case f1 and a lower case f2. The upper case f1 and lower case f2 are secured by locking to create the joint structure a of the flat cable c and the joint terminal b.
Another joint structure of the flat cable and the joint terminals as shown in FIG. 8 is disclosed in Japanese Paten Publn. 6-30270.
In FIG. 8, the first ends of a plurality of joint terminals b, i.e., the terminal plates b1, are arranged according to the arrangement pitch of conductors c2 whereas the second ends thereof, i.e. the connector terminals b2 are arranged to extend within the terminal chamber d. The terminal plates b1 and the connector terminals b2 are secured by insulating layers h1 and h2, respectively to create a terminal holder g.
The terminal plates b1 are welded to the conductor c2 by e.g. ultrasonic welding. A region e' where the terminal plates b1 and the conductors c2 are sandwiched and held by the upper case f1 and the lower case f2. The upper case f1 and lower case f2 are secured by locking to create the joint structure a' of the flat cable c and the joint terminals b.
The above two joint structures described above have the following drawbacks.
Since the connecting section e (e') is only sandwiched by both upper and lower cases f1 and f2, the mechanical strength of attaching the former to the latter is relatively low. The force for holding the connecting section e (e') by both cases f1 and f2 is also small. When the flat cable c is pulled, owing to the small holding force, pulling force is apt to concentrate on the connecting section. Thus, the connecting section e (e') may suffer from direct force so that it is destroyed. Further, since the upper case f1 and lower case f2 are secured by locking, when the flat cable c is pulled, both cases f1 and f2 may be loosed. This produces unexpected force which may destroy the connecting section e (e').